In the dialogue field "Work with:" enter the update site URL: http://download.lero.ie/updates/spl/

Select S2T2 Configurator from the software found on the update site

Important: Make sure that the checkbox "Contact all update sites during install to find required software" is selected

Confirm all further steps to finish wizard and restart Eclipse

If installation was successful you should be able to open feature models in fmprimitives format with the S2T2 Configurator within Eclipse (copy/load a *.fmprimitives file into Eclipse and select Open with... -> S2T2 Configurator in its context menu).